Source/JavaScriptCore/bytecode/Watchpoint.h - WebKit - Git at Google

 /*
  * Copyright (C) 2012-2016 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
  * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
  * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
  * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
  * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */

 #pragma once

 #include <wtf/Atomics.h>
 #include <wtf/FastMalloc.h>
 #include <wtf/Noncopyable.h>
 #include <wtf/PrintStream.h>
 #include <wtf/SentinelLinkedList.h>
 #include <wtf/ThreadSafeRefCounted.h>

 namespace JSC {

 class FireDetail {
     void* operator new(size_t) = delete;

 public:
     FireDetail()
     {
     }

     virtual ~FireDetail()
     {
     }

     virtual void dump(PrintStream&) const = 0;
 };

 class StringFireDetail : public FireDetail {
 public:
     StringFireDetail(const char* string)
         : m_string(string)
     {
     }

     void dump(PrintStream& out) const override;

 private:
     const char* m_string;
 };

 class WatchpointSet;

 class Watchpoint : public BasicRawSentinelNode<Watchpoint> {
     WTF_MAKE_NONCOPYABLE(Watchpoint);
     WTF_MAKE_FAST_ALLOCATED;
 public:
     Watchpoint()
     {
     }

     virtual ~Watchpoint();

 protected:
     virtual void fireInternal(const FireDetail&) = 0;

 private:
     friend class WatchpointSet;
     void fire(const FireDetail&);
 };

 enum WatchpointState {
     ClearWatchpoint,
     IsWatched,
     IsInvalidated
 };

 class InlineWatchpointSet;
 class VM;

 class WatchpointSet : public ThreadSafeRefCounted<WatchpointSet> {
     friend class LLIntOffsetsExtractor;
 public:
     JS_EXPORT_PRIVATE WatchpointSet(WatchpointState);

     // FIXME: In many cases, it would be amazing if this *did* fire the watchpoints. I suspect that
     // this might be hard to get right, but still, it might be awesome.
     JS_EXPORT_PRIVATE ~WatchpointSet(); // Note that this will not fire any of the watchpoints; if you need to know when a WatchpointSet dies then you need a separate mechanism for this.

     // Fast way of getting the state, which only works from the main thread.
     WatchpointState stateOnJSThread() const
     {
         return static_cast<WatchpointState>(m_state);
     }

     // It is safe to call this from another thread. It may return an old
     // state. Guarantees that if *first* read the state() of the thing being
     // watched and it returned IsWatched and *second* you actually read its
     // value then it's safe to assume that if the state being watched changes
     // then also the watchpoint state() will change to IsInvalidated.
     WatchpointState state() const
     {
         WTF::loadLoadFence();
         WatchpointState result = static_cast<WatchpointState>(m_state);
         WTF::loadLoadFence();
         return result;
     }

     // It is safe to call this from another thread.  It may return true
     // even if the set actually had been invalidated, but that ought to happen
     // only in the case of races, and should be rare. Guarantees that if you
     // call this after observing something that must imply that the set is
     // invalidated, then you will see this return false. This is ensured by
     // issuing a load-load fence prior to querying the state.
     bool isStillValid() const
     {
         return state() != IsInvalidated;
     }
     // Like isStillValid(), may be called from another thread.
     bool hasBeenInvalidated() const { return !isStillValid(); }

     // As a convenience, this will ignore 0. That's because code paths in the DFG
     // that create speculation watchpoints may choose to bail out if speculation
     // had already been terminated.
     void add(Watchpoint*);

     // Force the watchpoint set to behave as if it was being watched even if no
     // watchpoints have been installed. This will result in invalidation if the
     // watchpoint would have fired. That's a pretty good indication that you
     // probably don't want to set watchpoints, since we typically don't want to
     // set watchpoints that we believe will actually be fired.
     void startWatching()
     {
         ASSERT(m_state != IsInvalidated);
         if (m_state == IsWatched)
             return;
         WTF::storeStoreFence();
         m_state = IsWatched;
         WTF::storeStoreFence();
     }

     void fireAll(VM& vm, const FireDetail& detail)
     {
         if (LIKELY(m_state != IsWatched))
             return;
         fireAllSlow(vm, detail);
     }

     void fireAll(VM& vm, const char* reason)
     {
         if (LIKELY(m_state != IsWatched))
             return;
         fireAllSlow(vm, reason);
     }

     void touch(VM& vm, const FireDetail& detail)
     {
         if (state() == ClearWatchpoint)
             startWatching();
         else
             fireAll(vm, detail);
     }

     void touch(VM& vm, const char* reason)
     {
         touch(vm, StringFireDetail(reason));
     }

     void invalidate(VM& vm, const FireDetail& detail)
     {
         if (state() == IsWatched)
             fireAll(vm, detail);
         m_state = IsInvalidated;
     }

     void invalidate(VM& vm, const char* reason)
     {
         invalidate(vm, StringFireDetail(reason));
     }

     bool isBeingWatched() const
     {
         return m_setIsNotEmpty;
     }

     int8_t* addressOfState() { return &m_state; }
     static ptrdiff_t offsetOfState() { return OBJECT_OFFSETOF(WatchpointSet, m_state); }
     int8_t* addressOfSetIsNotEmpty() { return &m_setIsNotEmpty; }

     JS_EXPORT_PRIVATE void fireAllSlow(VM&, const FireDetail&); // Call only if you've checked isWatched.
     JS_EXPORT_PRIVATE void fireAllSlow(VM&, const char* reason); // Ditto.

 private:
     void fireAllWatchpoints(VM&, const FireDetail&);

     friend class InlineWatchpointSet;

     int8_t m_state;
     int8_t m_setIsNotEmpty;

     SentinelLinkedList<Watchpoint, BasicRawSentinelNode<Watchpoint>> m_set;
 };

 // InlineWatchpointSet is a low-overhead, non-copyable watchpoint set in which
 // it is not possible to quickly query whether it is being watched in a single
 // branch. There is a fairly simple tradeoff between WatchpointSet and
 // InlineWatchpointSet:
 //
 // Do you have to emit JIT code that rapidly tests whether the watchpoint set
 // is being watched?  If so, use WatchpointSet.
 //
 // Do you need multiple parties to have pointers to the same WatchpointSet?
 // If so, use WatchpointSet.
 //
 // Do you have to allocate a lot of watchpoint sets?  If so, use
 // InlineWatchpointSet unless you answered "yes" to the previous questions.
 //
 // InlineWatchpointSet will use just one pointer-width word of memory unless
 // you actually add watchpoints to it, in which case it internally inflates
 // to a pointer to a WatchpointSet, and transfers its state to the
 // WatchpointSet.

 class InlineWatchpointSet {
     WTF_MAKE_NONCOPYABLE(InlineWatchpointSet);
 public:
     InlineWatchpointSet(WatchpointState state)
         : m_data(encodeState(state))
     {
     }

     ~InlineWatchpointSet()
     {
         if (isThin())
             return;
         freeFat();
     }

     // Fast way of getting the state, which only works from the main thread.
     WatchpointState stateOnJSThread() const
     {
         uintptr_t data = m_data;
         if (isFat(data))
             return fat(data)->stateOnJSThread();
         return decodeState(data);
     }

     // It is safe to call this from another thread. It may return a prior state,
     // but that should be fine since you should only perform actions based on the
     // state if you also add a watchpoint.
     WatchpointState state() const
     {
         WTF::loadLoadFence();
         uintptr_t data = m_data;
         WTF::loadLoadFence();
         if (isFat(data))
             return fat(data)->state();
         return decodeState(data);
     }

     // It is safe to call this from another thread.  It may return false
     // even if the set actually had been invalidated, but that ought to happen
     // only in the case of races, and should be rare.
     bool hasBeenInvalidated() const
     {
         return state() == IsInvalidated;
     }

     // Like hasBeenInvalidated(), may be called from another thread.
     bool isStillValid() const
     {
         return !hasBeenInvalidated();
     }

     void add(Watchpoint*);

     void startWatching()
     {
         if (isFat()) {
             fat()->startWatching();
             return;
         }
         ASSERT(decodeState(m_data) != IsInvalidated);
         m_data = encodeState(IsWatched);
     }

     void fireAll(VM& vm, const FireDetail& detail)
     {
         if (isFat()) {
             fat()->fireAll(vm, detail);
             return;
         }
         if (decodeState(m_data) == ClearWatchpoint)
             return;
         m_data = encodeState(IsInvalidated);
         WTF::storeStoreFence();
     }

     void invalidate(VM& vm, const FireDetail& detail)
     {
         if (isFat())
             fat()->invalidate(vm, detail);
         else
             m_data = encodeState(IsInvalidated);
     }

     JS_EXPORT_PRIVATE void fireAll(VM&, const char* reason);

     void touch(VM& vm, const FireDetail& detail)
     {
         if (isFat()) {
             fat()->touch(vm, detail);
             return;
         }
         uintptr_t data = m_data;
         if (decodeState(data) == IsInvalidated)
             return;
         WTF::storeStoreFence();
         if (decodeState(data) == ClearWatchpoint)
             m_data = encodeState(IsWatched);
         else
             m_data = encodeState(IsInvalidated);
         WTF::storeStoreFence();
     }

     void touch(VM& vm, const char* reason)
     {
         touch(vm, StringFireDetail(reason));
     }

     // Note that for any watchpoint that is visible from the DFG, it would be incorrect to write code like:
     //
     // if (w.isBeingWatched())
     //     w.fireAll()
     //
     // Concurrently to this, the DFG could do:
     //
     // if (w.isStillValid())
     //     perform optimizations;
     // if (!w.isStillValid())
     //     retry compilation;
     //
     // Note that the DFG algorithm is widespread, and sound, because fireAll() and invalidate() will leave
     // the watchpoint in a !isStillValid() state. Hence, if fireAll() or invalidate() interleaved between
     // the first isStillValid() check and the second one, then it would simply cause the DFG to retry
     // compilation later.
     //
     // But, if you change some piece of state that the DFG might optimize for, but invalidate the
     // watchpoint by doing:
     //
     // if (w.isBeingWatched())
     //     w.fireAll()
     //
     // then the DFG would never know that you invalidated state between the two checks.
     //
     // There are two ways to work around this:
     //
     // - Call fireAll() without a isBeingWatched() check. Then, the DFG will know that the watchpoint has
     //   been invalidated when it does its second check.
     //
     // - Do not expose the watchpoint set to the DFG directly, and have your own way of validating whether
     //   the assumptions that the DFG thread used are still valid when the DFG code is installed.
     bool isBeingWatched() const
     {
         if (isFat())
             return fat()->isBeingWatched();
         return false;
     }

     // We expose this because sometimes a client knows its about to start
     // watching this InlineWatchpointSet, hence it'll become inflated regardless.
     // Such clients may find it useful to have a WatchpointSet* pointer, for example,
     // if they collect a Vector of WatchpointSet*.
     WatchpointSet* inflate()
     {
         if (LIKELY(isFat()))
             return fat();
         return inflateSlow();
     }

 private:
     static const uintptr_t IsThinFlag        = 1;
     static const uintptr_t StateMask         = 6;
     static const uintptr_t StateShift        = 1;

     static bool isThin(uintptr_t data) { return data & IsThinFlag; }
     static bool isFat(uintptr_t data) { return !isThin(data); }

     static WatchpointState decodeState(uintptr_t data)
     {
         ASSERT(isThin(data));
         return static_cast<WatchpointState>((data & StateMask) >> StateShift);
     }

     static uintptr_t encodeState(WatchpointState state)
     {
         return (static_cast<uintptr_t>(state) << StateShift) | IsThinFlag;
     }

     bool isThin() const { return isThin(m_data); }
     bool isFat() const { return isFat(m_data); };

     static WatchpointSet* fat(uintptr_t data)
     {
         return bitwise_cast<WatchpointSet*>(data);
     }

     WatchpointSet* fat()
     {
         ASSERT(isFat());
         return fat(m_data);
     }

     const WatchpointSet* fat() const
     {
         ASSERT(isFat());
         return fat(m_data);
     }

     JS_EXPORT_PRIVATE WatchpointSet* inflateSlow();
     JS_EXPORT_PRIVATE void freeFat();

     uintptr_t m_data;
 };

 } // namespace JSC
	/*
	* Copyright (C) 2012-2016 Apple Inc. All rights reserved.
	*
	* Redistribution and use in source and binary forms, with or without
	* modification, are permitted provided that the following conditions
	* are met:
	* 1. Redistributions of source code must retain the above copyright
	* notice, this list of conditions and the following disclaimer.
	* 2. Redistributions in binary form must reproduce the above copyright
	* notice, this list of conditions and the following disclaimer in the
	* documentation and/or other materials provided with the distribution.
	*
	* THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
	* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
	* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
	* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
	* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
	* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
	* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
	* OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
	* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
	* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
	*/

	#pragma once

	#include <wtf/Atomics.h>
	#include <wtf/FastMalloc.h>
	#include <wtf/Noncopyable.h>
	#include <wtf/PrintStream.h>
	#include <wtf/SentinelLinkedList.h>
	#include <wtf/ThreadSafeRefCounted.h>

	namespace JSC {

	class FireDetail {
	void* operator new(size_t) = delete;

	public:
	FireDetail()
	{
	}

	virtual ~FireDetail()
	{
	}

	virtual void dump(PrintStream&) const = 0;
	};

	class StringFireDetail : public FireDetail {
	public:
	StringFireDetail(const char* string)
	: m_string(string)
	{
	}

	void dump(PrintStream& out) const override;

	private:
	const char* m_string;
	};

	class WatchpointSet;

	class Watchpoint : public BasicRawSentinelNode<Watchpoint> {
	WTF_MAKE_NONCOPYABLE(Watchpoint);
	WTF_MAKE_FAST_ALLOCATED;
	public:
	Watchpoint()
	{
	}

	virtual ~Watchpoint();

	protected:
	virtual void fireInternal(const FireDetail&) = 0;

	private:
	friend class WatchpointSet;
	void fire(const FireDetail&);
	};

	enum WatchpointState {
	ClearWatchpoint,
	IsWatched,
	IsInvalidated
	};

	class InlineWatchpointSet;
	class VM;

	class WatchpointSet : public ThreadSafeRefCounted<WatchpointSet> {
	friend class LLIntOffsetsExtractor;
	public:
	JS_EXPORT_PRIVATE WatchpointSet(WatchpointState);

	// FIXME: In many cases, it would be amazing if this did fire the watchpoints. I suspect that
	// this might be hard to get right, but still, it might be awesome.
	JS_EXPORT_PRIVATE ~WatchpointSet(); // Note that this will not fire any of the watchpoints; if you need to know when a WatchpointSet dies then you need a separate mechanism for this.

	// Fast way of getting the state, which only works from the main thread.
	WatchpointState stateOnJSThread() const
	{
	return static_cast<WatchpointState>(m_state);
	}

	// It is safe to call this from another thread. It may return an old
	// state. Guarantees that if first read the state() of the thing being
	// watched and it returned IsWatched and second you actually read its
	// value then it's safe to assume that if the state being watched changes
	// then also the watchpoint state() will change to IsInvalidated.
	WatchpointState state() const
	{
	WTF::loadLoadFence();
	WatchpointState result = static_cast<WatchpointState>(m_state);
	WTF::loadLoadFence();
	return result;
	}

	// It is safe to call this from another thread. It may return true
	// even if the set actually had been invalidated, but that ought to happen
	// only in the case of races, and should be rare. Guarantees that if you
	// call this after observing something that must imply that the set is
	// invalidated, then you will see this return false. This is ensured by
	// issuing a load-load fence prior to querying the state.
	bool isStillValid() const
	{
	return state() != IsInvalidated;
	}
	// Like isStillValid(), may be called from another thread.
	bool hasBeenInvalidated() const { return !isStillValid(); }

	// As a convenience, this will ignore 0. That's because code paths in the DFG
	// that create speculation watchpoints may choose to bail out if speculation
	// had already been terminated.
	void add(Watchpoint*);

	// Force the watchpoint set to behave as if it was being watched even if no
	// watchpoints have been installed. This will result in invalidation if the
	// watchpoint would have fired. That's a pretty good indication that you
	// probably don't want to set watchpoints, since we typically don't want to
	// set watchpoints that we believe will actually be fired.
	void startWatching()
	{
	ASSERT(m_state != IsInvalidated);
	if (m_state == IsWatched)
	return;
	WTF::storeStoreFence();
	m_state = IsWatched;
	WTF::storeStoreFence();
	}

	void fireAll(VM& vm, const FireDetail& detail)
	{
	if (LIKELY(m_state != IsWatched))
	return;
	fireAllSlow(vm, detail);
	}

	void fireAll(VM& vm, const char* reason)
	{
	if (LIKELY(m_state != IsWatched))
	return;
	fireAllSlow(vm, reason);
	}

	void touch(VM& vm, const FireDetail& detail)
	{
	if (state() == ClearWatchpoint)
	startWatching();
	else
	fireAll(vm, detail);
	}

	void touch(VM& vm, const char* reason)
	{
	touch(vm, StringFireDetail(reason));
	}

	void invalidate(VM& vm, const FireDetail& detail)
	{
	if (state() == IsWatched)
	fireAll(vm, detail);
	m_state = IsInvalidated;
	}

	void invalidate(VM& vm, const char* reason)
	{
	invalidate(vm, StringFireDetail(reason));
	}

	bool isBeingWatched() const
	{
	return m_setIsNotEmpty;
	}

	int8_t* addressOfState() { return &m_state; }
	static ptrdiff_t offsetOfState() { return OBJECT_OFFSETOF(WatchpointSet, m_state); }
	int8_t* addressOfSetIsNotEmpty() { return &m_setIsNotEmpty; }

	JS_EXPORT_PRIVATE void fireAllSlow(VM&, const FireDetail&); // Call only if you've checked isWatched.
	JS_EXPORT_PRIVATE void fireAllSlow(VM&, const char* reason); // Ditto.

	private:
	void fireAllWatchpoints(VM&, const FireDetail&);

	friend class InlineWatchpointSet;

	int8_t m_state;
	int8_t m_setIsNotEmpty;

	SentinelLinkedList<Watchpoint, BasicRawSentinelNode<Watchpoint>> m_set;
	};

	// InlineWatchpointSet is a low-overhead, non-copyable watchpoint set in which
	// it is not possible to quickly query whether it is being watched in a single
	// branch. There is a fairly simple tradeoff between WatchpointSet and
	// InlineWatchpointSet:
	//
	// Do you have to emit JIT code that rapidly tests whether the watchpoint set
	// is being watched? If so, use WatchpointSet.
	//
	// Do you need multiple parties to have pointers to the same WatchpointSet?
	// If so, use WatchpointSet.
	//
	// Do you have to allocate a lot of watchpoint sets? If so, use
	// InlineWatchpointSet unless you answered "yes" to the previous questions.
	//
	// InlineWatchpointSet will use just one pointer-width word of memory unless
	// you actually add watchpoints to it, in which case it internally inflates
	// to a pointer to a WatchpointSet, and transfers its state to the
	// WatchpointSet.

	class InlineWatchpointSet {
	WTF_MAKE_NONCOPYABLE(InlineWatchpointSet);
	public:
	InlineWatchpointSet(WatchpointState state)
	: m_data(encodeState(state))
	{
	}

	~InlineWatchpointSet()
	{
	if (isThin())
	return;
	freeFat();
	}

	// Fast way of getting the state, which only works from the main thread.
	WatchpointState stateOnJSThread() const
	{
	uintptr_t data = m_data;
	if (isFat(data))
	return fat(data)->stateOnJSThread();
	return decodeState(data);
	}

	// It is safe to call this from another thread. It may return a prior state,
	// but that should be fine since you should only perform actions based on the
	// state if you also add a watchpoint.
	WatchpointState state() const
	{
	WTF::loadLoadFence();
	uintptr_t data = m_data;
	WTF::loadLoadFence();
	if (isFat(data))
	return fat(data)->state();
	return decodeState(data);
	}

	// It is safe to call this from another thread. It may return false
	// even if the set actually had been invalidated, but that ought to happen
	// only in the case of races, and should be rare.
	bool hasBeenInvalidated() const
	{
	return state() == IsInvalidated;
	}

	// Like hasBeenInvalidated(), may be called from another thread.
	bool isStillValid() const
	{
	return !hasBeenInvalidated();
	}

	void add(Watchpoint*);

	void startWatching()
	{
	if (isFat()) {
	fat()->startWatching();
	return;
	}
	ASSERT(decodeState(m_data) != IsInvalidated);
	m_data = encodeState(IsWatched);
	}

	void fireAll(VM& vm, const FireDetail& detail)
	{
	if (isFat()) {
	fat()->fireAll(vm, detail);
	return;
	}
	if (decodeState(m_data) == ClearWatchpoint)
	return;
	m_data = encodeState(IsInvalidated);
	WTF::storeStoreFence();
	}

	void invalidate(VM& vm, const FireDetail& detail)
	{
	if (isFat())
	fat()->invalidate(vm, detail);
	else
	m_data = encodeState(IsInvalidated);
	}

	JS_EXPORT_PRIVATE void fireAll(VM&, const char* reason);

	void touch(VM& vm, const FireDetail& detail)
	{
	if (isFat()) {
	fat()->touch(vm, detail);
	return;
	}
	uintptr_t data = m_data;
	if (decodeState(data) == IsInvalidated)
	return;
	WTF::storeStoreFence();
	if (decodeState(data) == ClearWatchpoint)
	m_data = encodeState(IsWatched);
	else
	m_data = encodeState(IsInvalidated);
	WTF::storeStoreFence();
	}

	void touch(VM& vm, const char* reason)
	{
	touch(vm, StringFireDetail(reason));
	}

	// Note that for any watchpoint that is visible from the DFG, it would be incorrect to write code like:
	//
	// if (w.isBeingWatched())
	// w.fireAll()
	//
	// Concurrently to this, the DFG could do:
	//
	// if (w.isStillValid())
	// perform optimizations;
	// if (!w.isStillValid())
	// retry compilation;
	//
	// Note that the DFG algorithm is widespread, and sound, because fireAll() and invalidate() will leave
	// the watchpoint in a !isStillValid() state. Hence, if fireAll() or invalidate() interleaved between
	// the first isStillValid() check and the second one, then it would simply cause the DFG to retry
	// compilation later.
	//
	// But, if you change some piece of state that the DFG might optimize for, but invalidate the
	// watchpoint by doing:
	//
	// if (w.isBeingWatched())
	// w.fireAll()
	//
	// then the DFG would never know that you invalidated state between the two checks.
	//
	// There are two ways to work around this:
	//
	// - Call fireAll() without a isBeingWatched() check. Then, the DFG will know that the watchpoint has
	// been invalidated when it does its second check.
	//
	// - Do not expose the watchpoint set to the DFG directly, and have your own way of validating whether
	// the assumptions that the DFG thread used are still valid when the DFG code is installed.
	bool isBeingWatched() const
	{
	if (isFat())
	return fat()->isBeingWatched();
	return false;
	}

	// We expose this because sometimes a client knows its about to start
	// watching this InlineWatchpointSet, hence it'll become inflated regardless.
	// Such clients may find it useful to have a WatchpointSet* pointer, for example,
	// if they collect a Vector of WatchpointSet*.
	WatchpointSet* inflate()
	{
	if (LIKELY(isFat()))
	return fat();
	return inflateSlow();
	}

	private:
	static const uintptr_t IsThinFlag = 1;
	static const uintptr_t StateMask = 6;
	static const uintptr_t StateShift = 1;

	static bool isThin(uintptr_t data) { return data & IsThinFlag; }
	static bool isFat(uintptr_t data) { return !isThin(data); }

	static WatchpointState decodeState(uintptr_t data)
	{
	ASSERT(isThin(data));
	return static_cast<WatchpointState>((data & StateMask) >> StateShift);
	}

	static uintptr_t encodeState(WatchpointState state)
	{
	return (static_cast<uintptr_t>(state) << StateShift) \| IsThinFlag;
	}

	bool isThin() const { return isThin(m_data); }
	bool isFat() const { return isFat(m_data); };

	static WatchpointSet* fat(uintptr_t data)
	{
	return bitwise_cast<WatchpointSet*>(data);
	}

	WatchpointSet* fat()
	{
	ASSERT(isFat());
	return fat(m_data);
	}

	const WatchpointSet* fat() const
	{
	ASSERT(isFat());
	return fat(m_data);
	}

	JS_EXPORT_PRIVATE WatchpointSet* inflateSlow();
	JS_EXPORT_PRIVATE void freeFat();

	uintptr_t m_data;
	};

	} // namespace JSC